Known apparatuses of this kind require a strong AC magnetic field in order to achieve an acceptable measuring signal. The magnetic field is generated either by means of an electromagnet, mechanically by means of a reluctance modulator (U.S. Pat. No. 3,240,051) or by means of a rotating magnetic system. A mechanic system can only operate at comparatively low frequencies, e.g. 10 Hz, and is thus sensitive to noise and vibrations. Higher frequencies can be achieved with an electromagnet, e.g. 100-200 Hz, which, however, requires a comparatively high electric power input.